Place 4-1/2 cups of the potatoes in a large saucepan and cover with water. Bring to a boil. Reduce heat; cover and cook for 10-15 minutes or until tender. Remove from the heat (do not drain). Mash potatoes; stir in butter.

In a 3-qt. slow cooker, combine the ham, celery, onion, garlic powder, paprika, pepper and remaining cubed potatoes. Stir in the mashed potato; top with cheese. Cover and cook on low for 5-6 hours or until potatoes and other vegetables are tender. Stir in the sour cream until blended. Thin soup with milk if desired.